1. Field of the Invention
This invention relates to the production of petroleum coke and more specifically to an improvement in petroleum coking drums for reducing coke slumping during the coke removal process.
2. Background Art
Petroleum coke is generally produced commercially by charging a heavy petroleum residuum stream to a coking drum at an elevated temperature typically from about 900.degree. F. to about 950.degree. F. until the drum is filled and the heavy petroleum residuum has been converted to petroleum coke. As the coke forms in the coking drum, the heavy petroleum residuum is thermally cracked into lighter hydrocarbons, which are recovered for further processing, and petroleum coke, which is subsequently recovered from the coking drum by the use of high-pressure water jets which are used to drill the petroleum coke from the coke drum. The coke typically falls into the coke pit from which it is recovered and passed to coke storage and thereafter to coke calcining to produce calcined petroleum coke.
Such a process is shown in FIG. 1. In FIG. 1, a heavy petroleum residuum stream from a vacuum flasher or other refinery source is passed to a fractionator 10 through a line 12. In fractionator 10, the heavy petroleum residuum is topped to produce a suitable feed stock for a coking drum and then passed through a line 14 to a heater 20 where it is heated to a suitable temperature, typically about 900.degree. F. to about 950.degree. F., and then passed through a line 22, a valve 24 and a line 26 to a coking drum 30 where petroleum coke is produced. Valve 24 is used to direct the heavy petroleum residuum to one of two or more coke drums (30 and 32) which are used alternately so that one may be used at all times to receive heavy petroleum residuum while the other is being emptied and the like. After the coke drum has been filled to a desired level with petroleum coker the introduction of new feed is stopped and the feed is switched to coke drum 32. During the production of the petroleum coke, lighter hydrocarbon materials are formed and are recovered through a line 34, passed through a valve 40, a line 42, a valve 46 and a line 48 to fractionator 10. A plurality of hydrocarbon streams and water may be recovered from fractionator 10 through lines 64, 66, 68 and 70. Normally, the products recovered from fractionator 10 are not finished refinery products but are fractions which are passed to further processing. Similarly, the water stream recovered from fractionator 10 at certain intervals during the operation of the coking drums may be mixed with light hydrocarbons which are desirably separated from the water vapor.
After the coke drum has been filled to a desired level, stripping steam at a high temperature and pressure is injected into the coking drum through a line 60 to strip volatile hydrocarbon components from the petroleum coke. The stream of stripped hydrocarbon materials and steam is passed to fractionator 10 where the heavier hydrocarbon materials are recovered and the steam and light hydrocarbons are passed to further treatment. After the stripping steam has removed a major portion of the volatile components from the petroleum coke, the coke is further cooled with additional steam supplied through line 60 for about one-half hour or until a suitably reduced temperature in the petroleum coke has been achieved. The petroleum coke is then cooled with water also supplied through line 60 to a temperature of about 200.degree. F. The cooling steam is passed from coke drum 30 through line 34, valve 40, line 42, valve 46 and line 50 to a blow-down drum 52 where water and light hydrocarbons are recovered from the mixture through a line 54 and passed to further processing and heavier hydrocarbons are recovered through a line 56 and passed to further processing, for instance in fractionator 10 or the like.
After the petroleum coke has been cooled to a suitable temperature, the injection of water is stopped and a top header 74 and a bottom header 76 are removed. The removal of these headers, especially the lower header, are operations that require care to ensure adequate safety for the operating personnel. When the top header is removed, quantities of steam and the like may be emitted. When bottom header 76 is removed, not only may quantities of steam be emitted but quantities of water which have been held up in coke drum 30 may be released along with possibly small quantities of coke. This hot water, hot steam and hot particles can be very hazardous to operating personnel. As a result the removal of bottom header 76 is generally accomplished by releasing the header and lowering it onto a header cart (not shown) which is used to move the header away from the bottom of coke drum 30. Thereafter, a bottom header 76 is removed and a pilot hole 90 (shown in FIG. 2) is cut through the coke by a high-pressure water jet mounted on a drillstem (not shown) through the center of the coke in drum 30. The drillstem is then retracted and thereafter the remaining coke in the drum is cut from the drum using high-pressure water jets which are also mounted on the drillstem. The removed coke falls through the pilot hole into a sump beneath coke drum 30 for recovery.
The top of a typical coking drum is shown in FIG. 3 and includes a flange 92, a top header 74 and an outlet line 94. Coke is generally accumulated in drum 30 to a desired height well below the top of the drum so that operating room is available for steaming, drilling the pilot hole and the like. Top header 74, as indicated, is removed when it is desired to remove the coke from the drum.
The operation of a petroleum coking process as described above is considered to be known to those skilled in the art and coking processes of this type are shown in U.S. Pat. No. 3,917,564, issued Nov. 4, 1975, to Meyers, U.S. Pat. No. 4,130,475, issued Dec. 19, 1978, to Cameron et al, U.S. Pat. No. 4,334,981, issued Jun. 15, 1982 to Holloway et al, U.S. Pat. No. 4,666,585, issued May 19, 1987, to Figgins et al, U.S. Pat. No. 4,874,505, issued Oct. 17, 1989, to Bartilucci et al, and U.S. Pat. No. 4,968,407, issued Nov. 6, 1990 to McGrath et al. These patents show coking processes including various modifications and variations thereof and are hereby incorporated in their entirety by reference.
The opening in the bottom of coke drum 30 is typically about ten to twelve feet in diameter as opposed to the opening in the top of the coke drum which is typically about three to four feet in diameter. As a result, there is a tendency in some instances, when the bottom header is removed, for the coke in drum 30 to sag or slump through the opening in the bottom of the coke drum, thereby resulting in difficulty in removing the bottom header from beneath coke drum 30. This results in a dangerous manual operation to chip the coke away to permit the removal of the bottom header and the like. As a result, a continuing effort has been directed to the development of a method for preventing slumping of the coke in coke drum 30 upon removal of bottom header 76.